I just read this article and found it to be really insightful.
It mostly talks about dogs in general. I do know that Yorkies lean a little bit more the the pancreatic side, so a diet lower in fat than a fat rich diet would seem to make more sense.
Here's the link and there are some other links that go to other great canine nutrition informaiton as well.
Are Dogs Carnivores… or Omnivores?